The field of the invention is that of the transmission of digital signals (or sampled and digitized analog signals) to one or more receivers. More specifically, the invention can be applied to transmission systems enabling the transmission of a multiplex that is generally (but not exclusively) formed by several independent source signals, according to a multiplex structure that can be redefined according to need, and implementing a time interleaving of the data elements that form these source signals.
A particular field of the invention is that of the transmission of signals that simultaneously implement a plurality of carrier frequencies, each encoded by distinct digital data elements.
Signals of this kind are generally designated by the term FDM (frequency division multiplex) signals. A particular example of these signals to which the invention can be applied is that of OFDM (orthogonal frequency division multiplex) signals.
An OFDM signal is used for example in the digital broadcasting system described for example in the French patent application FR-86 09622 filed on Jul. 2nd 1986 and in the document by M. Alard and R. Lassalle, "Principes de modulation et de codage canal en radiodiffusion numerique vcrs les mobiles" (Principles of channel modulation and encoding in digital broadcasting to moving bodies), Revue de l'U.E.R., No. 244, August 1987, pp. 168-190, known as the COFDM (coded orthogonal frequency division multiplex) system.
This COFDM system has been developed in particular within the framework of the European DAB (digital audio broadcasting) project. It is also a candidate for standardization for the terrestrial broadcasting of digital television.
The signals transmitted by such systems take the form of successive frames, each frame comprising all the carrier frequencies. Several source signals can therefore be conveyed simultaneously, each source signal being for example, at a given instant, associated with a set of frequencies. The term "source signal" is understood of course to mean both signals that are quite independent, corresponding to various applications (sound, images, telematics, data, etc.) and distinct signals (or fluxes) of one and the same application (for example the right-hand and left-hand channel for a stereophonic signal, subtitling, translation, images associated with a main signal, etc.).
It can easily be understood that it is desirable that the structuring of the frame into data channels (corresponding to each source signal) should be modifiable as a function of need. For example, it may be the case that a station may make transmission only occasionally, and may therefore not have need for transmission resources except at its transmission times. It may also happen that the needs relating to an application may vary in the course of time: for example with the stereophonic transmission of music and the monophonic transmission of speech, the addition of a translation sound signal, the transmission of an illustration, etc. Yet another example is that of radio-message networks, where needs constantly vary. More generally, the administrator of the network may, for one reason or another, wish to change the structure of the frames.
One technique used to manage the structuring in frames and its reconfiguration is described in the patent FR-90 16383 dated Dec. 19th 1990 filed on behalf of the same Applicants and entitled "System for the transmission of data by distribution in the time-frequency space with channel structuring". According to this technique, at least certain frames periodically include a description of the current structure. Thus, any receiver may have knowledge of this structure so as to extract the signal or signals pertaining to it therefrom.
This technique has been introduced into the DAB standard. In this standard, the frame is divided into sub-channels. This terminology shall be preserved hereinafter without however thereby restricting the scope of the invention. It must be noted that, on the contrary, the invention can be applied to any type of structuring. In particular, it is possible to provide for several levels of structuring (into channels and sub-channels, etc.), several types of sub-channels (transparent sub-channels or packet sub-channels for example), etc.
A change in structuring, or a multiplex reconfiguration, may be requested at any time (the DAB standard provides for a minimum period of 16 frames, for reasons explained hereinafter). One problem however arises during each reconfiguration, owing to the time interleaving.
Indeed it is not possible to change a structure instantaneously. There exists a transient state of N frames, N being the depth of the time interleaving. More specifically, if we consider the N frames of the transient phase according to a reconfiguration decision, it is observed that they have to be built partially according to the previous structuring (with digital elements interleaved prior to the reconfiguration command) and partially according to the new structuring (digital elements interleaved after the command).
Thus, in the case of the DAB, it is known that the time interleaving method leads to the introduction of a total delay of 15 frames (at 24 ms, giving 360 ms). This delay is distributed between transmission (interleaving) and reception (de-interleaving).
In fact, a given bit is delayed by p frames at transmission during the interleaving and by (15-p) frames at reception during the de-interleaving. The number p is determined by the rank of the bit, modulo 16, in the sub-channel.
To introduce these delays, it is necessary, for each sub-channel, to memorize the bits of this sub-channel corresponding to the 16 most recent frames. The interleaving memory therefore contains all the bits of all the sub-channels on a horizon of 16 frames.
The memory allocation chosen is identical to that of the frame (called a CIF frame) but with a depth of 16 frames. Since each sub-channel has a number of 64-bit capacity units (or CU) that is an integer, the memory must be shared with an elementary size of 64 bits.
FIG. 1 can be used to take account of this memory allocation for an example with three sub-channels, 11, 12, 13. The structure of the frame 14 comprises 864 CU. A memory compartment 16 of the memory array 17 can therefore be characterized by two numbers that shall be called i and j (column, row), i corresponding to the rank of the bit in the frame and varying from 0 to 55295, and j corresponding to the index of the CIF frame and varying from 0 to 15.
The bits of a sub-channel are written in this memory so as to provide for the time interleaving.
For example, in the case of the DAB standard, to generate the CIF frame with an index n, the bit with the rank i in the sub-channel shall be written in the box characterized by i and j=f(i), hence in the column with a rank i but on a row with a rank "j" depending on i or rather on the rest of the division of i by 16 (a CIF frame being characterized by 55296 bits written on the same row).
Table 1 gives the value of the row j as a function of the remainder of the division of i by 16 and of the index n of the frame.
TABLE 1 ______________________________________ Row in which the rank i Remainder of the division bit is written (frame to of i by 16 (rank i of which the rank i bit the bit in the frame) belongs) ______________________________________ 0 r 1 r-8 2 r-4 3 r-12 4 r-2 5 r-10 6 r-6 7 r-14 8 r-1 9 r-9 10 r-5 11 r-13 12 r-3 13 r-11 14 r-7 15 r-15 ______________________________________
The re-reading of the sub-channel is done simply row-wise (first row for the frame with an index r).
With regard to the time interleaving function, the dynamic reconfiguration of the multiplex signifies a change in allocation of the sub-channels in the CIF frame.
This may be a change in the position of one or more sub-channels in the CIF frame, without any change in the bit rate. It may also be a reduction in the bit rate, hence of the number of CUs for a sub-channel or on the contrary an increase in the bit rate. The last two events also prompt a modification of the allocation of CUs in the CIF frame.
This change is requested by the multiplexing equipment and may be interpreted by a microprocessor which knows, frame after frame, the allocation of the CUs in the CIF frame.
The DAB standard lays down that, in the CIF frame, the change should be instantaneous and that the CU should always consist of bits belonging to the same sub-channel.
If read/write operations are done as described here above, it means that, as soon as there is a change in allocation, the bits of the sub-channel will be written and read at the memory location corresponding to the new allocation.
After more than 16 frames, when this memory has been completely filled by the appropriate sub-channel, the re-reading of this memory part will contain only bits of this same sub-channel and, consequently, the interleaving function will be completely operational (the system will be in a "steady" or "normal" state).
However, during the 15 frames following the change in allocation, the location corresponding to the new allocation does, of course, contain bits of the sub-channel newly allocated to this part but also bits belonging to the sub-channel formerly allocated to this part of the memory. This leads to the formation of CUs containing bits of two different sub-channels. The signal thus transmitted is not compatible with the standard during this 15.times.24 ms=360 ms transmission which, to put it briefly, is due to the inertia of the storage of the 15 previous frames.
The N frames of the transient phase are therefore non-homogeneous and therefore not decodable. In a decoder, they correspond to a noise or passing parasitic phenomenon.
To limit this problem, it is possible, as soon as the reconfiguration instruction is received, to consider the building of frames in a new structure, namely with units of homogeneous capacity (belonging to one and the same channel). This technique however does not resolve the problem since these capacity units would be incomplete during the transient phase. Furthermore, this technique leads to a loss of the last elements of data interleaved according to the preceding structure.
Finally, this technique dictates a complex memory management system. For it would be necessary to write the data elements in an unoccupied memory zone. The drawback of this method is that, firstly, it requires a quantity of memory greater than 16.times.55296 bits, and secondly that it makes it necessary to manage this memory like a hard disk of a computer permanently having an image of the memory occupied and the memory available. This memory management may become very complicated since, for example, if a sub-channel should need to increase its bit rate, then the number of additional CUs would be assigned to a non-contiguous memory zone. After several reconfigurations, including various increases and reductions of bit rates, the interleaving memory risks being subdivided into a very large number of blocks that are alternately free and occupied.
By way of an example, FIG. 2 shows how a reduction of a sub-channel and an increase of another may lead to the need to have additional memory space, create free zones and partition the memory allocated to a given sub-channel. The previous structure 21 provides for three sub-channels 22.sub.1 to 22.sub.3 leading to the occupation corresponding to the memory array 23. During the configuration, the new structure 24 provides for a broadening of the sub-channel 22.sub.1, the consequent shifting of the sub-channel 22.sub.2 and the reduction of the sub-channel 22.sub.3.
Consequently, the data elements of the sub-channel 22.sub.1 are written at the former stipulated location 26 but also in a new available zone 25, the zone 27 being taken up by the sub-channel 22.sub.2. When the normal state is again reached, the memory array has a vacant zone 28 since the sub-channel 22.sub.3 is reduced.
It can therefore be seen that the management of the memory can soon become very complicated, for the allocation of the sub-channels in the interleaving memory is completely independent of the allocation of the sub-channels in the CIF. Furthermore, a sub-channel may be allocated to several memory blocks like a file in the hard disk of a computer.
At the end of a certain number of reconfiguration operations, it might even be necessary to reset the process in order to recompact the interleaving memory so as to prevent the management of an excessively large number of pointers, and this resetting would imply a temporary interruption of the programs.
It is an aim of the invention in particular to overcome these different drawbacks of the prior art.
More specifically, an aim of the invention is to provide a method for the dynamic reconfiguration of the structure of the frames of a transmission system, through which there appears neither any data loss and nor any disturbance of the transmission despite the implementation of a time interleaving operation.
In other words, the invention is aimed at providing a method of this kind capable of the management, without loss of information, of the transient phase due to the interleaving when a reconfiguration is requested.
Another aim of the invention is to provide a method of this kind that requires no increase in storage capacity whether at transmission or at reception. More specifically, an aim of the invention is to enable the implementation of the method with a memory capacity corresponding to the size of a frame to be transmitted multiplied by the depth of the interleaving.
Yet another aim of the invention is to limit the increase in complexity due to the implementation of the method. In particular, it is an aim of the invention to provide a method of this kind that can be laid out for example in the form of a simple logic system consuming few transistors, in standard semiconductor components providing for the reception of a digital signal.
Another aim of the invention is to provide a method of this kind, capable of accepting any type of modification of the structuring (the size, number and location of the sub-channels) without any limits whatsoever, regardless of the rate of reconfiguration.
It is also an object of the invention to provide a method of this kind that does not disturb the working of the receivers that do not contain it (apart from the transient phase of course).
Another aim of the invention is to provide receivers capable of receiving and decoding the signals produced by the method of the invention that are simple to implement and are low-cost in terms of design and production.